Electrochemical lithium ion intercalation in Li _0.5Ni _0.25TiOPO ₄ examined by in situ X-ray diffraction

The complex structural transformations of Li _0.5Ni _0.25TiOPO ₄ during electrochemical lithiation have been examined by in situ X-ray diffraction. During the first lithiation two structural changes take place: first a transition to a second monoclinic phase (a = 9.085(4), b = 8.414(5), c = 6.886(5), β = 99.85(4)) and secondly a transition to a third phase with limited long-range order. The third phase is held together by a network of corner sharing Ti-O octahedra and phosphate ions with disordered Ni-Li channels. During delithiation the third phase is partially transformed back to a slightly disordered original phase, Li _0.5Ni _0.25TiOPO ₄ without formation of the second intermediate phase. These phase transitions correspond well to the different voltage plateaus that this material shows during electrochemical cycling.